1. Field of the Invention
The present invention relates to a microscope employed for a cell incubator and a method of preventing dew condensation on an objective lens of such microscope.
2. Description of the Related Art
In a field of biology and the like, observations of behaviors of living cells of animals and plants, which are kept alive under an appropriate condition, are generally carried out for a functional analysis of a living organism.
A cell incubator is employed to keep the living cell alive under a constant environmental condition for the observation of behavior of the living cell.
One conventional technique for observing a cultured cell which can be kept alive in the cell incubator with a microscope is disclosed in Japanese Patent Application Laid-Open No. 2004-141143, entitled “Incubator for Observation with Microscope,” in which the cell incubator is placed on a stage of an inverted microscope for observation.
According to this technique, a cell culture space in which the cell in the incubator is placed is controlled as to maintain a constant condition, i.e., constant temperature, humidity, and CO2 concentration, for example, respectively at 37° C., 100%, and 5%. The cell is kept alive under such constant environmental condition inside the incubator and a time course thereof or the like is observed with the microscope.
Here, the space inside the incubator is not completely thermally insulated from outside air. For example, an objective lens is placed so that a lens barrel thereof is disposed at a side of outside air of the incubator, whereas a top lens is disposed in the vicinity of the cell inside the incubator.
If an objective lens with a short working distance (WD) is employed, a distance between a cover glass to which the cell is pasted and a surface of the top lens of the objective lens is approximately 0.1 to 0.2 millimeter (mm). When a general dry objective lens is employed, air is filled between the cover glass and the surface of the top lens. Hence, a certain effect of thermal insulation can be expected to be exerted. However, when an immersion objective lens is employed, water or oil is filled between the cover glass and the top lens, whereby the effect of thermal insulation is significantly degraded. More specifically, since the objective lens is exposed to the outside air merely via water or oil filled in a small space of approximately 0.1 to 0.2 mm, and the cover glass to which the cell is pasted is disposed relative to the objective lens, the cell is deprived of the heat by the objective lens via oil or water. Hence, especially when the temperature of the outside air is low, it is difficult to maintain the temperature of the cell at 37° C.
A technique to solve the above described problem is to add a heater for the objective lens, which is disclosed, for example, in Japanese Patent Application Laid-Open No. 2002-250869, entitled “Lens Heater and Heating Apparatus with Lens Heater for Microscopic Observation,” in which the objective lens is warmed up by heat emitted from a nichrome wire, and in Japanese Utility Model Application Laid-Open No. H7-36118, entitled “Microscope,” in which the immersion objective lens is warmed up to a predetermined temperature through a distribution of water of a predetermined temperature. Here, the cell is maintained at a predetermined temperature (37° C., for example), through the control of the heater which serves to warm up the cell culture space in the incubator, as well as the control of an objective lens heater which serves to warm up the objective lens.
The techniques as disclosed in the above cited documents have various problems.
Firstly, though the cell culture space of the incubator is maintained in an environment in the temperature of 37° C. and at 100% humidity, the temperature of the objective lens is approximately 20° C., i.e., room temperature. Hence, when the objective lens as such is brought closer to the cell for focusing, the temperature of the objective lens suddenly increases and is exposed to a highly humid environment. Due to the differential temperature here, dew is condensed on the surface of the top lens of the objective lens to cause mist. When the objective lens employed in the microscope has a high numerical aperture (NA), even a tiny amount of mist causes degradation of microscopic image, hampering a satisfactory observation. In addition, when the objective lens with a high NA and a short WD is employed in the inverted microscope, sometimes a hole is provided on a bottom surface of the incubator, since the incubator needs to be moved together with the cell on the stage to change a portion to be observed. With the structure as described above, however, the objective lens is subjected to an air in high temperature and high humidity leaked out from the cell culture space, and the top lens is easily misted. Further, since WD is short, the top lens is brought even closer to the highly humid cell, which is another reason for misting over of the top lens.
According to the techniques as disclosed in Japanese Patent Application Laid-Open No. 2002-250869 and Japanese Utility Model Application Laid-Open No. H7-36118, the dew condensation on the top lens is prevented through heating of the objective lens 1. In the apparatuses disclosed in Japanese Patent Application. Laid-Open No. 2002-250869, and Japanese Utility Model Laid-Open No. H7-36118, the temperature of the objective lens is controlled through heating so that the temperature of the cell is always maintained at 37° C. Though the temperature of the top lens of the objective lens is controlled to be close to 37° C., it may become lower than 37° C., for example, approximately 36.9° C. or lower, depending on the temperature inside the cell culture space of the incubator, the temperature outside, or the like. For example, the gas temperature of the cell culture space, the temperature of the cell, the temperature of the top lens of the objective lens, and the outside temperature may become stable, respectively, at 38° C., 37° C., 36° C., and 35° C.
Under such condition, since the humidity inside the cell culture space is 100%, when the temperature of the top lens is lower than the gas temperature inside the cell culture space even by 0.1° C., dew is sometimes condensed on an outer surface of the top lens, whereby the microscopic image taken by the objective lens is significantly degraded.
In general, in the objective lens, the top lens is fixed to a metal middle frame of the objective lens barrel via bonding or caulking, for example. Hence, when the top lens is exposed to the cell culture space with 100% humidity, water vapor inside the cell culture space may permeate through the bonded portion, or leaks out from a tiny gap of approximately a few μm in the caulked portion, whereby the leaked out water vapor is gradually accumulated inside the objective lens. When the internal temperature of the objective lens is substantially equal to the gas temperature of the cell culture space and approximately 37° C., inside of the objective lens gradually becomes 100% in humidity. If the temperature variation inside the objective lens is such that the temperature of some portion or location thereof is slightly lower than 37° C., dew condensation occurs at such portion. When dew is condensed on any of the lenses in the objective lens, the microscopic image is degraded similarly as described above hampering a satisfactory observation.
Further, the incubator is provided with a lid or a cover having a similar function to allow the observer to take out a case in which the cell is placed from the cell culture space, exchange, or supply a new case with a cell. When the lid or the door is opened/closed, the outside air of a low temperature may invade into the cell culture space. Then, the air touches the top lens of the objective lens to suddenly cool the objective lens, causing dew condensation from the water vapor accumulated inside the objective lens. Then, similarly as described above, the microscopic image is degraded and a satisfactory observation becomes unachievable.